There are various requirements for the surfaces of various metal materials; rust-preventive properties may be cited as one important characteristic. Iron and steel products are used in various applications such as structures, automobiles, ships, cans and the like; in such applications, there are strong requirements for rust-preventive properties. Conventionally, various types of rust-preventive agents such as water-soluble rust-preventive agents, vaporizable rust-preventive agents, oil-based rust-preventive agents and the like have been used to obtain rust-preventive properties in iron and steel products. Generally, water-soluble rust-preventive agents are used for the purpose of temporary short-term rust prevention, and are not used for long-term rust prevention. Furthermore, vaporizable rust-preventive agents show an inherent rust-preventive power in a sealed state. Oil-based rust-preventive agents have a relatively strong rust-preventive power, and are able to provide long-term rust prevention; such agents include liquid-form rust-preventive oils, viscous rust-preventive greases, and agents in which rust-preventive additives, film forming additives or the like are dissolved in volatile organic solvents. However, rust-preventive oils and rust-preventive greases show stickiness following a surface treatment, and cannot be used as undercoating materials such as coating films or the like. Furthermore, agents formed by dissolving rust-preventive additives, film forming additives or the like in volatile organic solvents do no manifest a sufficient rust-preventive effect unless a thick film thickness is formed.
Moreover, zinc-plated steel plates are commercially marketed for the purpose of suppressing rust on iron. However, such plates prevent the rust of iron by means of a sacrificial anti-corrosion effect of zinc, which has a greater tendency toward ionization than iron. In this case, white rust of the zinc layer is prevented by further subjecting the surface of the zinc plating layer to a chemical conversion treatment such as a chromate treatment or the like, so that the rust-preventive properties are greatly reinforced. However, although such a chromate treatment produces a high rust-preventive effect even in a thin coating film, there is a demand for a rust-preventive coating film to replace this chromate treatment because of environmental problems. Currently, tannic acid has been proposed as a rust-preventive agent for zinc-plated steel plates; at present, however, it cannot be said that this has a sufficient effect compared to a chromate treatment.
Aluminum or aluminum alloys are light in weight, and have therefore attracted attention in various fields. However, since an aluminum oxide coating film is formed on the surface, the adhesion is not sufficient even in cases where the aluminum is directly coated with a paint. Numerous methods have been proposed as chemical conversion treatment methods for paint primary coating or undercoating, and numerous patent applications have currently been filed. At the present time, however, chromate treatment methods constitute the mainstream in the United States and Japan. However, as in the case of the above-mentioned zinc-plated steel plates, chromate treatments are undesirable from the standpoint of the environment, and there is a demand for a non-chromate treatment. Moreover, methods in which a treatment is performed with an alkali metal salt aqueous solution of silicic acid for the purpose of endowing the metal surface with characteristics such as corrosion resistance, static resistance and the like have also been proposed. In such methods, however, the treatment temperature is high, an immersion in boiling water or an immersion in an acid is performed as an after-treatment, and the desired characteristics cannot be obtained unless the alkali metal is removed.